The market for LED lamps has grown exponentially as residential and commercial consumers make the change, from incandescent and halogen bulbs, to LED lighting. The typical reasons are better power efficiency and much longer lifetime. In addition to the benefit of saving energy, consumers also want the same features in LED lamps that are provided for the halogen bulbs; specifically, the ability to dim the light because it provides the desired ambience.
Technically, an LED is generally understood as a semiconductor device that generates light when electrical energy is applied to the device. Multiple LEDs can be formed into an array and powered as a unit.
LEDs are voltage sensitive devices. An LED must be supplied with a voltage that is above a threshold voltage and a current that is below the rating of the particular LED device. Generally, the current that is supplied to an LED is dependent exponentially on the voltage, referring to the Shockley diode equation. A small change in voltage can cause a large change in current. If the maximum voltage rating is exceeded by a small amount, the current rating can be exceeded by a large amount, potentially damaging the LED.
An LED driver or driving circuit is a type of power conversion circuit that delivers constant current instead of constant voltage. The typical LED driving circuit, or driver device, will convert a line voltage alternating current (“VAC”) to a direct current (“DC”).
LED dimming solutions generally include constant current reduction (“CCR”) or pulse-wave modulation (PWM) dimming. Constant current dimming generally involves linear adjustment of the current through the LEDs. Pulse-wave modulation will drive the LEDs at one current level, but will turn the LEDs on or off at a frequency that is generally greater than 120 Hz.
Dimming LED drivers often use 0-10 V control signals to control the dimming functions. Namely, the control signal varies between zero and ten volts. As a result, the controlled lighting scales its output so that at 10 V, the controlled light operates at 100% of its potential output, and at 0 V it operates at 0% output (i.e., “Off”) or a minimum dim level (i.e., 10%)
In the assignment of pins on a microchip in a PWM dimming solution, the normal function of the LED driver is to read the 0-10 volt analog input voltage on the microchip and assign it as a digital value representing the analog voltage reading. The value is then set as the PWM reference and used to adjust the pulse width modulation output of the microchip. Thus, the PWM reference is then used to control the light output of the luminaire attached to the driver.
However, this methodology of measuring the output and then adjusting the pulse width modulation output (PWM) accordingly can cause problems in the lower levels of PWM output in relation to the light output. When the PWM output is very low, the human eye can detect very small changes in light level (i.e., 1 mA). Thus, fluctuation or flickering of light can be visually perceived by the human eye at low pulse width modulation levels.
Although very complicated and advanced digital filters have been developed in digital dimming applications, the flickering problem can still persist at a difference between the digital output signal values of 151 and 150.999. The flickering can still be seen due to the fact that through the binary methods the value of 150.999 becomes 150. This problem cannot, however, be fixed through adjusting to a rounding methodology, such as floor or ceiling based rounding, which rounds to the nearest integer either up or down. Therefore, the flickering problem is still prevalent at the digital output signal values between 150 to 150.001.
To address this problem, more processor intensive methods have been developed by some manufacturers. The drawback of these methods is that the implementation requires too much memory and thus requires a more expensive microchip. Another disadvantage is that these methods do not account for small average changes of the 0-10V line, which will cause changes of the PWM output.
Therefore, there remains a need for a low voltage solution to mitigate flicker. There also remains a need for system and method that provides a light load on the CPU while eliminating visual fluctuation of light output at low pulse width modulation levels.